Increasing concerns regarding traditional energy sources have prompted investigation of alternative, renewable sources of energy. Wave energy is a renewable energy source and countries with extensive coastlines and strong prevailing winds could produce considerable quantities of electricity from wave power.
Wave energy refers to the energy of ocean surface waves and the capture of that energy for the purpose of electricity generation. In general, the larger the wave, the more energy it contains, and therefore, the more energy that can be obtained from it. Specifically, the amount of energy which may be obtained from waves is determined by wave height, wave speed, wavelength, and water density.
Several types of devices may be used to capture wave energy. All of these devices work on a similar principle. The wave force acts on a moveable absorbing member, which reacts against a fixed point. The fixed point may be a land or sea-bed based structure, or another moveable, but force-resisting, structure. The wave force results in oscillatory motion of the absorbing member and the product of wave force and corresponding motion represents the converted energy.
There are several disadvantages associated with known energy absorbing devices. Extreme waves (i.e. exceptionally large waves with respect to the current wave state, or rapidly changing waves) can occur during otherwise benign wave states. Such large waves can cause an excessive force to be exerted on the linkage or coupling between the moveable member and the fixed point. This can result in breakage of the coupling, particularly in devices with no natural damping, such as linear energy converters. Accordingly, these devices have poor survivability, even in normal wave conditions. A further disadvantage associated with known wave energy converters is poor efficiency of energy capture. Typical devices are capable of capturing wave energy only over a relatively narrow range of wave frequencies and energy states. While more advanced devices can tune their response to enable them to optimize energy capture from any given wave state, such slow tuning usually only delivers a good response to the average power spectrum of that wave state. Few devices can respond rapidly enough to the individual frequencies within a single sea state.
An object of the invention is to provide a damping structure for a wave energy conversion (WEC) device that automatically counteracts or dampens any extreme wave forces. Another object of the invention is to provide a damping structure for a WEC device having improved efficiency of energy capture. A further object of the invention is to provide a damping structure for a WEC device that allows additional energy capture over a wide range of wave frequencies. A further object of the invention is to provide a damping structure for a WEC that allows the WEC to maintain an optimum alignment to the wave.
By way of background to the mooring application of this technology, it is noted that vessels and other sea-based devices such as fish farms, floating docks, oil rigs and floating wind farms are typically moored to fixed structures such as piers, quays or the seabed using mooring lines or hawsers. Wave energy conversion devices are typically moored in a similar manner.
Traditional mooring lines are usually made from synthetic materials, such as nylon or Kevlar. Typically, nylon mooring lines are quite elastic, which allows excess stress to be spread over a number of lines. However, nylon lines can only deliver small elongations of the order of 10%. Mooring lines may also be made from wire rope, which is extremely strong, but difficult to handle and maintain. Lines may also be made from a combination of wire rope and synthetic materials, in which case the line is referred to as a hawser.
However, these mooring solutions that are suitable for deep water or dock mooring are not suitable for low scope or small footprint mooring applications, where some devices, particularly renewable energy devices, need to operate. The “scope” of a mooring is the length of the mooring per unit of water depth. The “footprint” of a mooring is the seabed area occupied by the mooring. The problem lies in the relationship between the size of the waves, drift lengths and/or tidal changes, which are encountered in these environments and the inability of traditional mooring systems to flex with the forces and extension such conditions apply to the mooring, without resorting to large footprints or over-engineered solutions. Each mooring line has a finite breaking point or breaking limit. The higher the breaking limit, the greater the diameter or the higher the grade of material required, and thus the higher the cost of the mooring.
In certain environments, wave heights, drift lengths or tidal changes can easily exceed 25% of the water depth. For example, in non-sheltered ocean locations, wave heights can often exceed 10 meters in water depths of 30 to 40 meters. Tides cause changes in the depth of marine and estuarine water bodies and produce oscillating currents known as tidal streams. Tidal cycles last approximately 12 hours and 25 minutes in most locations and the tidal cycles involves the following sea level changes. Over several hours, water flows in one direction, known as flood flow, reaches a maximum height, known as high tide, and then lowers or falls off as water flows in another (not necessarily opposite) direction known as ebb tide until a low tide level is reached. Moorings system must be able to cope with this tidal turning. In tidal flow regions, that is, where a moored body is acted on by tidal streams or tidal turning, the drift forces can pull the mooring sizeable distances in one direction (horizontally) and then the other as the tide changes. In tidal barrage regions, that is, where there is a change in water depth due to tides, the tidal height can change by a few meters in shallow waters. Under any of these conditions, a mooring system needs to be flexible enough to allow for the device to ride the changes without requiring a significant footprint. Failure to achieve this results in significant loads being applied to the mooring system, which must either be designed for (which may result in overengineering of the mooring system) or the system risks breakages. The elasticity of nylon lines is not sufficient for these mooring applications, for example at a seabed depth of 30 meters, in regions where wave heights may be in the region of 10 meters.
One type of mooring used for certain applications is the catenary mooring. A catenary mooring comprises a free hanging line or cable, running horizontal to the seabed. The restoring force of the mooring line is primarily generated by the hanging weight and pretension in the line. An example of a prior art catenary mooring system is shown in FIG. 23, which illustrates that, as the water depth increases, the weight of chain acting on the floater increases and this can result in large resistive forces being exerted on the floater. Due to the horizontal load reacting nature of the conventional drag embedded anchors which are used with catenary systems, the scope of the cable must therefore be chosen such that the cable is never entirely picked up from the seabed for the given environmental conditions. As shown in FIG. 23, when the water depth is the same order of magnitude as large waves (i.e. depths of 20 m) the length of chain required to deal with changes in water depth of 40-60 m is very large. Normally a scope of three suffices, but in shallower, exposed areas scopes of more than five are frequently required. This is often inefficient and takes up a lot of seabed around the device and results in very high costs for the mooring system.
Another drawback of this type of system is that, in order to deal with large waves, the chain or cable lifts as the water depth increases and the floater moves both vertically and horizontally to a new position. Thus, a large space envelope is required to allow horizontal movement as water depths rise. This restricts both the density of floating bodies (e.g. floating platforms) that can be positioned within an area and also the accuracy to which those bodies can be positioned. A further disadvantage of the catenary system is fatigue, as the mooring lines tend to wear at the seabed touch down point.
Elastomeric mooring solutions are provided by a number of companies, including
Supflex®, Seaflex® and Hazelett Marine. The elastic properties of the Hazelett device absorb the peak loads and maintain a lower steady pull on the vessel or device. Under extreme loading, it may elongate up to 300%. The Seaflex® rubber hawser can withstand a force of drag greater than 10 kN and more than 100% elongation to allow the mooring to take care of natural and artificial water level fluctuations.
These passive elastomeric material solutions are becoming popular in near shore and dock mooring applications. They provide a number of advantages over traditional mooring solutions by allowing a flexible component in the mooring system to stretch with the heave and surge of the vessel or device. They also cause less seabed damage, as additional slackness can be built into the mooring system. However, these mooring systems are principally designed to prevent drift of vessels and are not designed to provide low scope, small footprint performance in deeper waters. These current elastomeric solutions work well where the change in height is small with respect to the depth of water in which the mooring is used, such as in-harbour pontoons, where wave heights are low with respect to water depth, and in estuaries, where tidal changes in water height are low. While they provide a natural non-linear stress strain response to applied wave forces, they do not deliver the performance and response curves required for more challenging mooring environments. In order to achieve the level of performance required for these applications, a relatively large scope, that is, length per unit of depth and a large seabed footprint are required. This means that more material, or higher-grade material, must be used, thereby increasing cost.
Typically, these elastomeric solutions comprise a multi-strand elastomeric component. The number of strands in the component may be varied in order to vary the damping response achieved. However, the response of the component to applied forces varies only in scale, and the basic response achieved remains the same. Thus, the response may only be tailored to one particular sea state or environmental loading (i.e. a fixed height to depth or current to depth ratio). In deeper of faster waters, the component is likely to snap due to excessive ratio change.
Ideally, a deep sea mooring system needs to be adaptable to the sea states at the location at which it is placed and so it must adjust to the applied forces from the waves over very short time periods. Ideally, the mooring system is self-adjusting so that risk of failure in harsh environments is reduced. Ideally, the mooring system should absorb load forces at the lowest possible breaking limit. It should also be cost-effective.
International Application Publication No. WO 96/27055 describes a hysteretic damping apparatus and method which uses one or more tension elements fabricated from shape memory alloy to cycle through a superelastic stress-strain hysteresis. The damping apparatus may be designed to have a selected stroke or force capacity by adjusting the length, thickness and number of the tension elements. The tension elements may be in the form of wire loops or bands and can be used to damp movement of structures such as offshore platforms subject to wave movement.
There are a number of disadvantages associated with this damping apparatus. First of all, this is a pure damping system which is concerned only with dissipation of energy. In a wave energy environment, this device would very quickly overheat and would be unable to dissipate the energy that deep sea waves contain. This apparatus is also unsuitable for any large scope mooring applications, since a large amount of heat is generated in dissipating such large quantities of energy. Additionally, the shape memory alloy materials used are usually unsuitable for a marine environment.